The broad long term goals of this research are to better understand the molecular genetic mechanisms underlying neuromuscular disease using a novel mouse mutation as an experimental model. Muscular dystrophies include a diverse group of genetically heterogeneous disorders characterized by progressive muscle weakness and wasting that leads to severe disability and often premature death. There is a need to learn more about pathogenesis of the diseases and translate this knowledge into effective treatments. Toward this goal, we propose to study the mechanism of pathogenesis in the mdm mutant mouse, a novel model of progressive muscular dystrophy that functionally links the enormous Titin (Ttn) gene to the limb-girdle muscular dystrophy type 2A (LGMD2A) cysteine protease calpain 3 (Capn3). We have genetically mapped and identified the mdm mutation as a complex rearrangement that results in a small in-frame deletion within a putative CAPN3-interacting domain of TTN. The mdm mouse may also serve as a genetic model for human tibial muscular dystrophy (TMD) which maps to the TTN locus at 2q31. This is the first demonstration that mutations in Ttn are associated with muscular dystrophy and provides a novel animal model to test for functional interactions between these two disease genes. [unreadable] [unreadable] The steps we will take to elucidate the roles of titin and calpain 3 in muscle cell degeneration will be to 1) test the hypothesis that calpain 3 interactions with titin are disrupted by the mdm mutation, 2) test the alternate hypotheses that the progressive mdm muscular dystrophy is due to either reduced CAPN3 levels or aberrant activation of the CAPN3 protease, and 3) generate a Ttn-null allele by gene targeting and an allelic series of muscular dystrophy mutations at the Ttn locus using a sensitized ENU mutagenesis screen. Thus, the mdm mutant mouse provides a unique tool for understanding molecular pathways causing muscular dystrophy and may reveal entry points in which to intervene in the disease process. [unreadable] [unreadable]